1. Field of the Invention
The present invention relates to gradient power source apparatus for use in magnetic resonance imaging (MRI) apparatus which supplies a gradient coil with a current having a desired waveform to thereby cause the gradient coil to produce a gradient magnetic field with a desired waveform.
2. Description of the Related Art
As one of magnetic resonance imaging techniques an echo-planar technique which permits ultra high-speed scan has been developed in recent years. The echo-planar technique will replace the conventional spin-echo technique. With such an ultra high-speed imaging technique, such a pulse sequence as shown in FIG. 1 is used. In this pulse sequence, a readout gradient field Gr is several times as strong as that used in the spin-echo technique. An ultra high-speed MRI apparatus requires a high current of the order of 500 to 600 amperes to flow through a gradient coil having considerably high inductance. Moreover the high current needs to be switched between positive and negative polarities at a high speed.
As a prior art of a gradient power source apparatus for carrying out an ultra-high-speed-scan pulse sequence there is known a "gradient current speed-up circuit for high-speed NMR imaging system" which is disclosed U.S. Pat. No. 4,961,054. This is a kind of inverter, and its basic arrangement is illustrated in FIG. 2. A gradient coil 2 represented by a series combination of inductance Lg and resistance Rg has an end connected in common to ends of switching devices SW1 and SW4 and the other end connected in common to ends of switching devices SW2 and SW3. The other ends of the switching devices SW1 and SW3 are connected together to the positive terminal of a constant-voltage source 6 through a choke coil 4 represented by a series combination of inductance Lc and resistance Rc. The other ends of the switching devices SW2 and SW4 are connected together to the negative terminal (ground terminal) of the voltage source 6. That is, the switching devices SW1 to SW4 construct a bridge circuit. As the switching devices SW1 to SW4, transistors, thyristors, gate turn-off thyristors (GTO) or the like can be used. The switching devices SW1 to SW4 are subjected to on/off control by control signals from a sequence controller (not shown) incorporated into MRI apparatus, thereby carrying out such a pulse sequence as shown in FIG. 1. Snubber circuits S used for protection are connected in parallel with the respective switching devices SW1 to SW4.
The operation of the gradient power source apparatus shown in FIG. 2 for producing, for example, the readout gradient magnetic field Gr shown in FIG. 1 will be described with reference to FIG. 3, which is a timing diagram illustrating on and off times of the switching devices SW1 to SW4, and waveforms of a source voltage Vin, a source current Iin, and a gradient coil current Ig.
At time t=0, the switching devices SW1 and SW4 are turned on and then the apparatus waits for the source current Iin to reach a predetermined value Ip (t=t1). At time t=t1, the switching devices SW3 and SW4 are turned on, and the switching devices SW1 and SW2 are turned off, so that a current Ig flows through the gradient coil 2. At time t=t2, the switching devices SW1 and SW2 are turned on, and the switching devices SW3 and SW4 are turned off, so that the direction of current flow through the gradient coil 2 is reversed. Repeating the on/off switching of the switching devices SW1 to SW4 in accordance with the timing diagram of FIG. 3 permits a rectangular-wave current Ig having such a flat top as the readout gradient field Gr shown in FIG. 1 has to flow through the gradient coil 2.
Since the inductance Lg of the gradient coil 2 is considerably high, however, the waveform of the coil current Ig may be so degraded that its flat top cannot be kept. In order to avoid the waveform degradation of the gradient coil current Ig, the choke coil 4 is connected between the power source 6 and the bridge-connected switching devices. The choke coil 4 has inductance Lc which is five to twenty times higher than the inductance Lg of the gradient coil 2, thus permitting the source current Iin to be kept substantially constant.
With such an arrangement, however, the connection of the choke coil 4 having high inductance Lc (usually 5 to 20 millihenries) requires to wait until the source current Iin reaches a predetermined current value Ip at t=1 from when the power is turned on at t=0. During the interval t=0 to t=t1, magnetic resonance imaging cannot be performed. That is, a waiting time of t1 is required from when the power is turned on until data acquisition is started, which offsets an advantage of ultra high-speed MRI apparatus that a large number of imaging scans can be accomplished within a short period of time. If, on the other hand, current were continued to flow between imaging scans so as to eliminate the waiting time, power consumption would increase because of a high current on the order of 500 to 600 amperes.
Moreover, the choke coil, which has high inductance and permits a high current flow, is very great in size and weight.
Furthermore, keeping the source current Iin substantially constant requires to store high magnetic energy in the choke coil 4, which increases power consumption. The magnetic energy Wc (in joules) is represented by EQU Wc=(1/2)LcIp.sup.2 ( 1)
where Lc is the inductance of the choke coil 4, and Ip is a current flowing through the choke coil 4 which equals a maximum value of the gradient coil current Ig.
Equation (1) shows that the higher the choke coil inductance Lc becomes, the higher the magnetic energy Wc to be stored becomes, and thus power consumption increases.
If the choke coil inductance Lc were decreased so as to shorten the waiting time and reduce the power consumption, the waveform of the gradient coil current Ig would deteriorate. Non-rectangular coil current waveforms would degrade MRI image quality.
It is required to keep a peak value of the gradient coil current substantially constant. When the switching is made at regular intervals as shown in FIG. 3, the peak value can be kept constant. If, however, the switching were made at irregular intervals to regulate echo acquisition times, the peak value could not be kept constant because of decay due to loss in the snubber circuits.
Another prior art, as shown in FIG. 4, adds capacitors C1 and C2 and diodes D1 to D4 to the arrangement of FIG. 2 for the purpose of regulating a reversing time (from t3 to t4 in FIG. 5) of the gradient coil current. FIG. 5 is a timing diagram illustrating the operation of this prior art.
However, a problem with the prior art is that the waveform of the gradient coil current Ig shown in FIG. 5 will be distorted by charging and discharging of the capacitors C1 and C2. This problem will be solved by further increasing the inductance of the choke coil 4. However, this not only increases the power consumption and lengthens the waiting time as described above but also makes the choke coil itself large.
Still another solution is to connect a resistor in parallel or series with each of the capacitors C1 and C2, thereby consuming extra energy. However, a problem arises in that power consumption increases.
As shown in FIG. 6A, even if a rectangular current is caused to flow through the gradient coil 2 to thereby produce a rectangularly pulsed gradient field, the waveform of a composite magnetic field, within a static-magnetic-field forming superconducting magnet, of the gradient field and a magnetic field component due to an eddy current generated in a metal cylinder of the superconducting magnet may be non-rectangular. Heretofore, in order to compensate for such waveform distortion, an eddy current compensating component is superimposed upon the gradient coil current as shown in FIG. 6B so that a composite magnetic field can have a rectangular waveform. However, a current type inverter will not generate a current having such a waveform as shown in FIG. 6B, failing to compensate for waveform distortion due to eddy current.